A dark mode FTP client for Windows
FTP by Zaweba has Dark Theme and Light Theme on its View menu, so the file panes and code editor can be easy on the eyes late in the evening.

Dark mode comes built into FTP by Zaweba, an FTP, FTPS and SFTP client for Windows. Open the View menu and choose Dark Theme; choose Light Theme to go back. It changes the look of the app you work in, where the file panes and the code editor take up most of your day.
People who look after websites often do it at the edges of the day, after the shop has closed or before the office opens. A bright white window in a dim room is tiring, and a dark theme is one of the simplest comforts an app can offer.
Switching themes
The View menu holds both themes. Pick Dark Theme and the window changes; pick Light Theme to return. Use whichever suits the room and the hour, and change as often as you like.
There is nothing else to set up and no account to sign in to. FTP by Zaweba is free software, and the theme is just one of its options.
Your website keeps its own colours
The theme changes FTP by Zaweba, not your site. When you open Live Preview (Ctrl+Shift+P) beside the code, the page shows in its own colours, exactly as a visitor's browser would draw it. That is what you want: a preview that turned your white page dark would hide the very things you are checking.
If your site has its own dark design for visitors who prefer it, you can work on it in the editor like any other CSS, and the preview updates as you type HTML or edit a stylesheet.
Making long sessions comfortable
A dark theme is one part of a comfortable setup. A few other things in FTP by Zaweba help on a long evening of edits.
Pop the preview out into its own window and put it on a second monitor. Its open state, width, window and device size are remembered, so it comes back where you left it. The code then gets the whole of the main window.
Use the editor's own tools to keep the screen calm: code folding hides long blocks you aren't working on, word wrap stops long lines running off the side, and Ctrl+P opens a file by name so you aren't scanning the tree for it.
Turn on Upload on Save for the site, if that is how you like to work, so a save sends the file without another step. Conflict warnings still apply, so if someone else changed the server copy since you last fetched it, you are told before anything is overwritten.
The other options while you are there
File > Options holds a few settings worth a look once. You can set how many files transfer at once, from 1 to 10 with 3 as the default; shared hosts limit connections, and FTP by Zaweba carries on with fewer if a server refuses more. You can set limits for the local version history, which keeps previous versions of files on your PC before they are overwritten, by number per file, total size and maximum file size. And you can set your name for activity logs, used if you switch on the per-site log of who changed what.
Why it matters more than it seems
A theme is a small thing, but small discomforts add up over hundreds of hours. Being able to choose the look that suits you, in a client that also edits, previews, syncs and tidies your server, means one less reason to keep a second program open. FTP by Zaweba is built for people who build and look after websites on ordinary web hosting, and comfortable evenings are part of that job.